Nacreous pigments and their optical effects are well known. As a result of the interference phenomenon in which certain wave-lengths are reinforced and others are cancelled, colors ranging from pearl to green can be produced by controlling the thickness of the coating on various substrates. These colors are obtained by reflection and, when the pigment is viewed by transmission, the complementary color is obtained.
The interference colors are not strong and, in many instances, it is necessary to add absorption pigments in order to enhance the play of colors. The absorption pigment alters both the reflection color which is viewed and the transmission color. If too great a concentration of absorption pigments is used, the pearly or nacreous quality of the product is lost. The absorption pigments, although very finely divided, scatter light, and a diminution of the nacreous luster results. Therefore, the amount of absorption pigment which can be added in many instances is limited by the decrease in luster which can be tolerated.
There are several methods which are used to add absorption pigments to nacreous pigments. If the absorption pigment is very finely divided, a simple blending can suffice. However, very finely divided pigments which are acceptable for cosmetic applications, a primary use of nacreous pigments, are not available. If the absorption pigment particles are very large and used as such, they scatter light and detract from the luster effect.
Another problem which is encountered when blending a finely divided absorption pigment with a nacreous pigment is that a separation can take place in certain formulations with the finely divided pigment migrating and resulting in a non-homogenous product. For example, blending a finely divided Iron Blue with a blue reflecting titanium dioxide coated nacreous pigment results in a pigment in which both the reflection color and absorption color are blue. This pigment, however, suffers from the fact that the finely divided Iron Blue is not held on the surfaces of the nacreous pigment and migration with its resulting discoloration can occur when dispersed in different vehicles.
It is obviously advantageous if the absorption pigment can be made highly adherent to the surfaces of the nacreous pigment and, in this way, prevent migration of color under use conditions. For example, precipitating a layer of chromium oxide hydroxide onto the surfaces of a green reflecting titanium dioxide coated mica nacreous pigment followed by calcination results in the green absorbing pigment being held on the surface of the substrate and preventing migration. Such a method, however, results in a color of low purity, and very high concentrations of chromium oxide are needed in order to produce intense green colors.
In many applications, especially in the field of cosmetics, a precise color is needed if the desired effect is to be produced. In many cases, various shades of green are desired, ranging from the blue-green to the yellow-green. Precipitation of hydrous chromic oxide does not permit shading, which is extremely important in cosmetics.
Iron Blue, which is also known as Prussion Blue, and Berlin Blue, is acceptable for use in cosmetic preparations. By mixing the absorbing Iron Blue pigment with an appropriately colored nacreous pigment, a green product can be obtained. It has been pointed out above that merely blending Iron Blue with the nacreous pigment results in a product in which the Iron Blue is not held on the surfaces of the pigment and migration with its resulting discoloration occurs. In U.S. Pat. No. 3,951,679, it is pointed out that the conventional precipitation of absorption pigments onto nacreous pigments is unsuitable for a precipitation of Iron Blue because the latter forms colloidal deposits during direct precipitation, which adhere only partially or not at all on the flakes of the nacreous pigment. This leads to non-reproducible batches, to considerable difficulties during filtering, and most particularly, a lack of nacreous effect.
In order to cause Iron Blue to adhere to the surfaces of the nacreous pigment, U.S. Pat. No. 3,951,679 employs the device of coating an Iron Blue precursor on the nacreous pigment and then converting the precursor to the Iron Blue. This is accomplished by coating the starting mica flakes with ferrous hexacyanoferrate (II), either by direct precipitation or by first precipitating a ferrous salt and then reacting the salt with a hexacyanoferrate (II), followed by oxidation to Iron Blue. Alternatively, the mica flakes are coated with a ferric salt, either directly or by first coating with a ferrous salt which is oxidized to the ferric salt, followed by reacting the precipitated ferric salt with hexacyanoferrate (II) to form the Iron Blue. The direct precipitation of a ferric salt is not preferred because it is difficult to coat the starting pigment as a result of the tendency of precipitates of trivalent iron compounds to assume the colloidal form. A third alternative described is the coating of the mica flakes with a ferrous salt followed by reaction with a hexacyanoferrate (III). Whichever method is employed, it is indicated that the precipitation conditions are adjusted conventionally so that the precipitate is immediately and quantitatively deposited on the mica flakes and, in that connection, it is important that an excess of metallic ions be avoided.
As is apparent from the foregoing description, a method of directly precipitating Iron Blue onto the surfaces of a nacreous pigment has not heretofore been described, and it is accordingly the object of this invention to provide such a method. This and other objects of the invention will become apparent to those of skill in this art from the following detailed description.